This invention relates a fuel injection control system for a diesel engine for causing fuel to be sprayed into a combustion chamber of a cylinder of the engine a plurality of times at least at a point of time near top dead center of a compression stroke of the cylinder.
A known fuel injection control system of this type is adapted to spray fuel during an intake stroke and at a point of time near top dead center of a compression stroke as disclosed, for example, in Japanese Unexamined Patent Publication No. 9-158810. In this control system, a part of the fuel is first preliminarily sprayed by a fuel injector during the intake stroke of a cylinder and sufficiently mixed with an intake air in a combustion chamber to form a preliminary lean air-fuel mixture, and the remaining fuel is sprayed at a point of time near top dead center of the compression stroke of the cylinder to cause an autoignition. In this way, the preliminary lean air-fuel mixture is satisfactorily burnt, to reduce an exhaust of smoke while suppressing production of NOx.
In the above prior art diesel engine, production of NOx is further suppressed by retarding a timing of the primary injection performed at a point of time near top dead center of the compression stroke of the cylinder. In addition to this, it has been proposed to split the primary injection and perform it twice in order to prevent a part of the lean air-fuel mixture from being exhausted while being unburnt. By splitting the primary injection, the fuel is more fully burnt, so that the diesel engine can be highly efficiently operated with low emissions of toxic exhaust components.
However, if the primary injection is retarded as in the prior art, the completion timing of the primary injection is retarded. Further, if the primary injection is split and performed a plurality of times, the completion timing of the primary injection is retarded by the intermission intervals between the split injections. Thus, under an operating condition where a larger output is required for an engine and, accordingly, a larger amount of fuel needs to be sprayed, most of the fuel sprayed at a final stage of the primary injection is exhausted without being sufficiently burnt, resulting in a poor fuel consumption and a bad influence on environments due to a larger ratio of unburnt gases to burnt gases.
For example, in a low engine rotation speed operating condition of the engine such as an idle operating condition, a fuel injection pressure by an injector is relatively set low in order to cause a spray of the fuel to suitably spread in the combustion chamber. If the engine enters an accelerating state in this condition, a fuel injection amount has to be sharply increased despite a low fuel injection pressure. This extends a valve opening period of the injector as a matter of course, with the result that the above problems are likely to occur.
It is an object of the present invention to provide a fuel injection control system for a diesel engine which is free from the problems residing in the prior art.
According to an aspect of the invention, whether or not an injection completion timing is later than a point of time corresponding to a specified crank angle is judged when fuel sprayed during the primary injection is split and sprayed at different timings near top dead center of a compression stroke of each cylinder. There is provided a corrector for correcting a mode of the fuel injection by the split injection controller when the primary injection completion timing is judged to be later than the point of time corresponding to the specified crank angle.
It may be preferable to correct at least one of the number of split injections, an injection intermission interval and a fuel injection pressure to advance the injection completion timing when the injection completion timing is judged to be later. Alternatively, it may be preferable to reduce the amount of fuel to be sprayed during the primary injection for correction to advance the injection completion timing, and perform an auxiliary fuel injection separately when the injection completion timing is judged to be later.
Accordingly, it can be assuredly accomplished to prevent deterioration of fuel consumption and increase in emissions resulting from an excessive retardation of an injection completion timing.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments/examples.